JupyterLabに拡張機能を追加したい

拡張機能をインストールしようとして、いくつかの拡張機能で試してみましたが、
JupyterLabのGUIからもコマンドプロンプトからやってもエラーが起きてしまいます。
開発環境の構築に問題があったのではないかと考えているのですが、問題の切り分け方や調べ方などでも結構ですので、アドバイスをいただければありがたいです。

発生している問題・エラーメッセージ

【ブラウザGUIから実行した場合のエラーメッセージ】

【コマンドプロンプトCUIから実行した場合のエラーメッセージ】

C:\Users\###########>jupyter labextension install @jupyter-widgets/jupyterlab-manager
Traceback (most recent call last):
  File "E:\App\Anaconda3\lib\site-packages\jupyterlab_server\server.py", line 20, in <module>
    from notebook.notebookapp import aliases, flags, NotebookApp as ServerApp
  File "E:\App\Anaconda3\lib\site-packages\notebook\notebookapp.py", line 51, in <module>
    from zmq.eventloop import ioloop
  File "E:\App\Anaconda3\lib\site-packages\zmq\__init__.py", line 50, in <module>
    from zmq import backend
  File "E:\App\Anaconda3\lib\site-packages\zmq\backend\__init__.py", line 40, in <module>
    reraise(*exc_info)
  File "E:\App\Anaconda3\lib\site-packages\zmq\utils\sixcerpt.py", line 34, in reraise
    raise value
  File "E:\App\Anaconda3\lib\site-packages\zmq\backend\__init__.py", line 27, in <module>
    _ns = select_backend(first)
  File "E:\App\Anaconda3\lib\site-packages\zmq\backend\select.py", line 28, in select_backend
    mod = __import__(name, fromlist=public_api)
  File "E:\App\Anaconda3\lib\site-packages\zmq\backend\cython\__init__.py", line 6, in <module>
    from . import (constants, error, message, context,
ImportError: DLL load failed while importing error: 指定されたモジュールが見つかりません。

During handling of the above exception, another exception occurred:

Traceback (most recent call last):
  File "E:\App\Anaconda3\Scripts\jupyter-labextension-script.py", line 5, in <module>
    from jupyterlab.labextensions import main
  File "E:\App\Anaconda3\lib\site-packages\jupyterlab\labextensions.py", line 16, in <module>
    from .commands import (
  File "E:\App\Anaconda3\lib\site-packages\jupyterlab\commands.py", line 29, in <module>
    from jupyterlab_server.process import which, Process, WatchHelper, list2cmdline
  File "E:\App\Anaconda3\lib\site-packages\jupyterlab_server\__init__.py", line 4, in <module>
    from .app import LabServerApp
  File "E:\App\Anaconda3\lib\site-packages\jupyterlab_server\app.py", line 9, in <module>
    from .server import ServerApp
  File "E:\App\Anaconda3\lib\site-packages\jupyterlab_server\server.py", line 26, in <module>
    from jupyter_server.base.handlers import (                          # noqa
ModuleNotFoundError: No module named 'jupyter_server'

該当のソースコード

jupyter labextension install @jupyter-widgets/jupyterlab-manager

試したこと

そもそもjupyter lab自体が正しく動いていないのかとも思い、以下のコマンドも試してみました。

(1) jupyter labextension list
(2) jupyter lab

【結果：(1)(3)ともに同じ】

C:\Users\#####>jupyter labextension list
Traceback (most recent call last):
  File "E:\App\Anaconda3\lib\site-packages\jupyterlab_server\server.py", line 20, in <module>
    from notebook.notebookapp import aliases, flags, NotebookApp as ServerApp
  File "E:\App\Anaconda3\lib\site-packages\notebook\notebookapp.py", line 51, in <module>
    from zmq.eventloop import ioloop
  File "E:\App\Anaconda3\lib\site-packages\zmq\__init__.py", line 50, in <module>
    from zmq import backend
  File "E:\App\Anaconda3\lib\site-packages\zmq\backend\__init__.py", line 40, in <module>
    reraise(*exc_info)
  File "E:\App\Anaconda3\lib\site-packages\zmq\utils\sixcerpt.py", line 34, in reraise
    raise value
  File "E:\App\Anaconda3\lib\site-packages\zmq\backend\__init__.py", line 27, in <module>
    _ns = select_backend(first)
  File "E:\App\Anaconda3\lib\site-packages\zmq\backend\select.py", line 28, in select_backend
    mod = __import__(name, fromlist=public_api)
  File "E:\App\Anaconda3\lib\site-packages\zmq\backend\cython\__init__.py", line 6, in <module>
    from . import (constants, error, message, context,
ImportError: DLL load failed while importing error: 指定されたモジュールが見つかりません。

During handling of the above exception, another exception occurred:

Traceback (most recent call last):
  File "E:\App\Anaconda3\Scripts\jupyter-labextension-script.py", line 5, in <module>
    from jupyterlab.labextensions import main
  File "E:\App\Anaconda3\lib\site-packages\jupyterlab\labextensions.py", line 16, in <module>
    from .commands import (
  File "E:\App\Anaconda3\lib\site-packages\jupyterlab\commands.py", line 29, in <module>
    from jupyterlab_server.process import which, Process, WatchHelper, list2cmdline
  File "E:\App\Anaconda3\lib\site-packages\jupyterlab_server\__init__.py", line 4, in <module>
    from .app import LabServerApp
  File "E:\App\Anaconda3\lib\site-packages\jupyterlab_server\app.py", line 9, in <module>
    from .server import ServerApp
  File "E:\App\Anaconda3\lib\site-packages\jupyterlab_server\server.py", line 26, in <module>
    from jupyter_server.base.handlers import (                          # noqa
ModuleNotFoundError: No module named 'jupyter_server'

補足情報（バージョン）

####開発環境
Windows 10 Pro

Python 3.8.5
conda 4.9.2
jupyter core : 4.6.3
jupyter-notebook : 6.1.4
qtconsole : 4.7.7
ipython : 7.19.0
ipykernel : not installed
jupyter client : not installed
jupyter lab : 2.2.6
nbconvert : not installed
ipywidgets : not installed
nbformat : 5.0.8
traitlets : 5.0.5